Compositional zoning in garnet and kinetics of metamorphic crystallization

The kinetics of zoned garnet porphyroblast growth is exemplified in a sample of garnet-staurolite-biotite schist from the northern Ladoga region. The diffusion-controlled porphyroblast growth was accompanied by a decrease in the kinetic coefficient during phase reactions. Even at insignificant (1–2°C) thermal overstepping, the leading role of diffusion as a factor that controls kinetics of porphyroblast growth in medium-grade metapelites is consistent with the parameters of metamorphic crystallization: T = 500–650°C, t = 1 Ma; D _A1 ^app = 10^?14 cm²/s, L = 0.2–0.6 cm, r = 1–3 mm, ΔC _Al = 1.5 × 10^?4–1.5 × 10^?3 mol/cm³.